Carbon dioxide is conventionally obtained as a gaseous by-product from the production of ammonia or hydrogen as well as from fermentation plants. The by-product generally contains carbon dioxide. It is known to convert the gaseous by-product into pure liquid carbon dioxide by distillation at recoveries exceeding 94% by weight.
Conventional distillation columns used for producing liquid carbon dioxide typically operate at about a pressure of 260 psia and a column condenser temperature of about -25.degree. F. The waste gas removed from the top of the column as an overhead stream, under these conditions contains about 75% by volume of carbon dioxide. Accordingly, the amount of carbon dioxide lost as waste is about triple the amount of impurities in the feed. It therefore follows that carbon dioxide recovery decreases significantly as the concentration of carbon dioxide in the feed decreases.
Efforts at treating low concentration carbon dioxide feeds have focused on decreasing the carbon dioxide concentration in the column overhead. This has been done by either increasing the column operating pressure or by decreasing the column condenser temperature. However, both of these methods suffer from significant disadvantages.
When the column pressure is increased, the solubility of inert impurities such as oxygen and nitrogen in the liquid carbon dioxide product increases.
In addition, the power needed for the refrigeration cycle increases and adds to the cost of the process. Further, at significantly higher pressures and in the presence of impurities such as oxygen, phase behavior limitations make it impossible to produce pure carbon dioxide by vapor-liquid separation.
Similarly, there are significant disadvantages in reducing the column condenser temperature. In conventional systems, much of the equipment includlng the distillation column is made of carbon steel. The lowest temperature which carbon steel can withstand is -35.degree. F. Stainless steel such as Type 304 can be used at temperatures as low as -50.degree. F., but at a significant increase in material cost.
By decreasing the temperature of the column from -35.degree. F. to -50.degree. F. it is possible to increase the recovery of carbon dioxide from a given feed concentration. Similarly, it is possible to increase the carbon dioxide recovery at a given temperature if the pressure is increased. Nonetheless, the carbon dioxide recovery from these conventional systems is below 94% by weight if the concentration of the carbon dioxide feed is less than about 89% by volume.
Pressure swing adsorption has been used as an alternative to solvent absorption for separating carbon dioxide. For example, S. Sircar et al., U.S. Pat. No. 4,077,779 discloses a process for separating methane from carbon dioxide.
M. Duckett et al., U.S. Pat. No. 4,639,257 discloses a process for producing liquid carbon dioxide in which a membrane separator is used to separate gaseous carbon dioxide from components of a waste stream. The system is said to be useful to treat low concentration feed sources by compressing the feed gas to high pressures of at least 200 psia.
The membrane separation disclosed in Duckett et al. is limited to removing only those impurities which are significantly less permeable through the membrane than carbon dioxide, and are not suitable for the separation of carbon dioxide from impurities which are more permeable. Thus the membrane separation disclosed by Duckett et al. can not be used to separate hydrogen or helium from carbon dioxide. Since the membrane must also be highly selective for carbon dioxide, impurities such as oxygen, which are slightly less permeable than carbon dioxide, are not readily separated by membrane separation systems.
Further, dual compression is required when the membranes are first installed to separate the feed. Thus, in accordance with Duckett et al., the feed gas must be compressed before membrane separation and then the carbon dioxide-rich permeate must be recompressed prior to recycling to the distillation column. These requirements add significantly to the cost of the process and significantly limit its application.